QGIS crashes when QgsRendererV2.symbolForFeature() is called from Python

[qgib]

Author Name: Minoru Akagi (@minorua)
Original Redmine Issue: 10096
Affected QGIS version: master

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With graduated styled layer, the following code crashes QGIS. It works in 2.2 without any problems.

layer = iface.activeLayer()
renderer = layer.rendererV2()

f = QgsFeature()
layer.getFeatures().nextFeature(f)
s = renderer.symbolForFeature(f)

With categorized styled layer, symbolForFeature() returns None. It returns a subclass object of QgsSymbolV2 correctly in 2.2.

Crash doesn't seem to occur if the graduated styled layer has been loaded through a project file, and a subclass object of QgsSymbolV2 is returned correctly, but with categorized styled layer, it still returns None.

This method is useful for plugins which export data with styles.

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• symbolForFeature.zip (Minoru Akagi)

[qgib]

Author Name: Matthias Kuhn (@m-kuhn)

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As a workaround for the crash: embed your calls within renderer.startRender() and renderer.stopRender() to have the renderer in a state where it is able to evaluate expressions.

In the long term it would however be nice to have somebody who knows this code to have a look at the lifetime of the expression which crashes.

[qgib]

Author Name: Minoru Akagi (@minorua)

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Matthias Kuhn wrote:

As a workaround for the crash: embed your calls within renderer.startRender() and renderer.stopRender() to have the renderer in a state where it is able to evaluate expressions.

Works fine with the workaround. Thanks, Matthias!

[qgib]

Author Name: Jürgen Fischer (@jef-n)

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Fixed in changeset "dcc9d39a273eebc54f996af088daca23b31c0efb".

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• status_id was changed from Open to Closed

[qgib]

Author Name: Alexander Bruy (@alexbruy)

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Still true in latest master.

Seems this method does not work as expected for almost all renderers:

• QgsSingleSymbolRendererV2 — returns feature symbol
• QgsCategorizedSymbolRendererV2 — returns None and in debug output I have message saying "there are no hashed symbols!!!"
• QgsGraduatedSymbolRendererV2 — just crashes QGIS
• QgsRuleBasedRendererV2 — returns None

Tested with point shapefile and same code as in initial post.

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• status_id was changed from Closed to Reopened

[qgib]

Author Name: Matthias Kuhn (@m-kuhn)

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I think the workaround should be accepted as solution and stated in the API documentation according to [1]

The only thing left to fix is the crash in graduatedsymbolrenderer. The last "fix" was also just about the crash and not about changing the behavior to be compatible with calls out of startRender() / stopRender() context.

[1] http://permalink.gmane.org/gmane.comp.gis.qgis.devel/32841

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• status_id was changed from Reopened to Feedback

[qgib]

Author Name: Matthias Kuhn (@m-kuhn)

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No crash here.
If it's still reproducable, please reopen.

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• resolution was changed from to fixed/implemented
• status_id was changed from Feedback to Closed